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Mechanica

MPT1327 Trunked Radio Systems

I thought that the best way of showing the principles of MPT1327 trunked
radio systems was with an example. The example I have chosen is
the a local councils MPT1327 system which is standard enough so that
what I describe can be applied to just about any MPT1327 system.

So lets say you own an FTrunk decoder , live in this councils area and tune to
177.1625 MHz which is this systems control channel. You will hear the rapid
warbling noise of the data and if you set the decoder running you will see something like this ..

This sites ID is 117.

09:17:05 Prefix#0 Unit 768 Registering
09:17:14 Prefix#0 Unit 807 Calling Unit 852 Goto 178.6125 MHz
09:17:28 Prefix#0 Unit 842 Calling Unit 807 Goto 179.4625 MHz
09:17:51 Prefix#0 Unit 376 Calling Unit 7800 Goto 180.2375 MHz

What does all this mean ..

The first line gives the sites ID (sometimes called the SYSID) in this case 117. This is the identity number of the base station that is being monitored. Each base station in a network needs a different identity number to identify it to
The mobile radios in the network. These numbers are not allocated by the
government but are decided by the radio company who build the network.

Next the line ..

09:17:05 Prefix#0 Unit 768 Registering

Now a little basic MPT1327 theory. Each MPT1327 radio on a network needs a unique identity number (kind of like a telephone number). In MPT1327 this
identity is made up of 2 numbers. These are the prefix which is a number between 0 and 127 and the ident which is a number between 1 and 8100. So in the line above we are seeing a radio with the prefix 0 and ident 768 register with the network. Registering is a brief burst of data a radio sends to the the base station either when it is first turned on or when it comes into range of
the control channel. When the base station computer receives it then it knows
that radio is within its range so it can accept its calls. However a base station
doesn't always let a mobile radio register as sometimes you will see something like this ..

09:29:08 Prefix#0 Unit 903 Registration Rejected

Here a mobile radio with the prefix 0 and ident 903 has tried to register. The base has decided that either it does not know about this radio or possibly that
this radio has been reported stolen. So it tells it that its registration has been rejected and this radio is unable to use this trunked system.

Now you have seen how a mobile radio registers lets see how FTrunk shows
when a radio wants to a make a voice call ..

09:17:14 Prefix#0 Unit 807 Calling Unit 852 Goto 178.6125 MHz

Here radio prefix 0 and ident 807 has called another radio which has a prefix of 0 and ident 852. The base station controller looks for a free voice channel
finds that 178.6125 MHz is free so sends the radios to that channel for their
voice conversation. If you had a scanner capable of computer control that was
supported by FTrunk it would be automatically tuned to this frequency so you could monitor the voice call.

The next line in the log is ..

09:17:28 Prefix#0 Unit 842 Calling Unit 807 Goto 179.4625 MHz

Here radio prefix 0 and ident 842 is calling radio prefix 0 and ident 807. The base controller has checked and found that 178.6125 MHz is still busy with the first voice call so instead sends both radios to another voice channel 179.4625 MHz for their voice call.

This next line looks the same as the last 2 but is in fact slightly different ..

09:17:51 Prefix#0 Unit 376 Calling Unit 7800 Goto 180.2375 MHz

Notice how the second ident 7800 is much higher than the first one 376 ?
This probably means the second ident is something called a group ident. What this means is that if someone calls ident 7800 they won't be talking to a single radio but a group of radios where any member of the group can hear the conversation and join in if needed just like an old style repeater. An FTrunk user could confirm that ident 7800 is a group ident by checking the rest of his logs. As a group ident isn't a real radio it cannot call anyone so the ident 7800 will never appear as the first ident in such a voice call command. In addition a group ident will never register.

Sometimes you will see another kind of voice call command that looks like this ..

09:54:29 Prefix#0 PSTN Gateway Calling Unit 512 Goto 178.6125 MHz

This trunked system is also connected to the public telephone system and some of its radios can be called or make calls on it just like mobile phones. Here someone has used the telephone system (or PSTN Public Switched Telephone Network) to call radio prefix 0 and ident 512. The base station controller handles this like a normal voice call and has sent the radio to 178.6125 MHz where the call will take place.

FTrunk also allows the user to customise the display. So lets say that after listening you discover that prefix 0 ident 450 is Park Ranger South and ident 451 is Park Ranger North then you can easily customise the program so instead of showing ..

Prefix#0 Unit 451 Calling Unit 450 Goto 179.4625 MHz

It would show ..

Prefix#0 Park Ranger North Calling Park Ranger South Goto 179.4625 MHz

Now MPT1327 has lots more facilities it can be used for sending data such as a vehicles position or text messages between radios (just a like text messages on a mobile phone). But I want to keep this as a brief introduction
To MPT1327 and not get to complicated for now.

I hope you enjoyed this.

If you want to find out more about the FTrunk decoder click here .

2002

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